Physicochemical analysis of mangrove soil: Muthurajawela - Negombo wetland complex, Sri Lanka

Abstract

Mangroves are unique and valuable ecosystems. This study examined some selected physicochemical parameters of mangrove soil and the mangrove distribution according to the salinity and pH levels at the Muthurajawela-Negombo wetland complex. Twenty sampling sites were selected based on the judgmental sampling technique by creating a fishnet grid of 30 m × 30 m. Soil samples were taken from 0 - 15, 15 - 30 and 30 - 45 cm depths from the surface. Soil temperature was measured using a soil thermometer, and other physicochemical parameters such as pH, salinity, and soil organic matter (SOM) were analyzed using standard laboratory methods. Further, a vegetation survey was carried out to study the distribution of mangrove plants in relation to the soil pH and salinity. Results showed that temperature varied spatially from 25.2 °C to 30.0 °C, with the highest temperature recorded in the topsoil layer. Soil pH and salinity spatially varied from 5.39 to 8.31 and 0.56% to 8.83%, respectively. Soil pH did not significantly vary with depth (p > 0.05), while salinity increased with the increasing depth. Soil organic matter spatially ranged from 2.56 to 15.70% and also increased with depth. Soils with high salinity tend to accumulate SOM. The correlation analysis revealed a lower positive relationship between soil pH and temperature (r = 0.15; p < 0.05), and a positive relationship between salinity and SOM (r = 0.316; p < 0.05). Rhizophora apiculata, Rhizophora mucronata and Avicennia marina were associated more in soils with high salinity (3.72 - 7.15%) and neutral to weakly alkaline pH. Bruguiera gymnorrhiza was more prevalent in soils with higher salinity (7.69 - 8.83%) and lower pH, while Lumnitzera racemosa was found in acidic to slightly alkaline pH but with low salinity (1.35 - 1.92%) soils. Sonneratia caseolaris was recorded in soils with the lowest salinity (0.83 - 1.04%). The findings of the study contribute to decision-making on the conservation and restoration of mangrove projects.